The invention relates to optical fiber attenuators, and more particularly to a variable optical attenuator.
In optical systems, certain optical signals often need to be attenuated. As such, optical systems often include optical attenuators. A particular type of attenuator is a variable optical attenuator, which can vary the amount of attenuation. Such attenuators have a variety of potential uses in optical systems. For example, variable optical attenuators can be used to compensate for variable input strengths to achieve a constant output strength, or to compensate for variable path length attenuation to produce equal strength signals for signals that travel different paths. Alternatively, variable optical attenuators can be used to compensate for variable input strengths to achieve desired but differing lower signal strengths.
Some objectives in the field of variable optical attenuators are to provide such attenuators in a manner that is both cost effective and highly reliable. One prior solution, for example, is illustrated in FIGS. 1A, 1B and 1C. A variable optical attenuator 10 is provided with a housing 12 that supports a motor 14. A drive shaft 16 extends from the motor 14 passes through, and mounts together with, a device 18 for converting the rotational motion to linear motion, (e.g., rack and pinion, threaded screw and nut, worm gear, cam, and the like). A rectangular shaped filter 22 mounts on one end of the device 18. An input optical fiber 24 enters the housing 12 and terminates with an input collimator 26. Also provided is an output collimator 28, which is in optical communication with output optical fiber 30. The output optical fiber 30 exits the housing 12 at a second end, thus transmitting any light signals out of the attenuator 10. In addition, a potentiometer 32 mounts, for example, at a distal end of the drive shaft 16 extending from the motor 14. The potentiometer 32 indicates a rotational position of the drive shaft 16.
The rectangular filter 22 is illustrated in FIG. 1B. The rectangular filter 22 is a neutral density filter with a linearly increasing gradient. As a light signal travels through input optical fiber 24 and input collimator 26 the light signal passes through rectangular filter 22 before entering output collimator 28 and output optical fiber 30. The motor 14 activates the device 18 for converting rotational motion to linear motion and linearly slides the rectangular filter 22 to a desired attenuation position. This form of optical attenuator 10 has a significant number of moving parts. The device 18, depending on its particular configuration, can experience an amount of backlash or play, which makes specific placement of the rectangular filter 22 and the subsequent attenuation level more difficult to achieve. There is also a concern that the backlash or play can be affected by vibrations from surrounding machinery, which might ultimately cause creep and a subsequent unintentional change in attenuation level.
FIG. 1C illustrates a graphical representation of a level of attenuation versus amount of linear motion on the part of the rectangular filter 22. As can be seen, this relationship is substantially linear.
A second conventional solution to variable optical attenuation, for example, is illustrated in FIGS. 1D, 1E, and 1F. As shown, an optical attenuator 34 has a housing 36 that supports a motor 38. The motor 38 has a drive shaft 40 extending therefrom. A circular filter 42 mounts on the drive shaft 40 of the motor 38 such that the drive shaft 40 passes through a center point of the circular filter 42. An input optical fiber 44 enters the housing 36 at one end and mounts to input collimator 46. In addition, an output collimator 48 is in optical communication with an output optical fiber 50. The output optical fiber 50 extends out a second end of the housing 36. Once again, a potentiometer 52 is provided at a distal end of the drive shaft 40 to indicate the rotational position of the drive shaft 40 and the circular filter 42.
In this version of variable optical attenuator 34, input optical fiber 44 provides a light signal to input collimator 46. The light signal passes through circular filter 42 and enters output collimator 48 to subsequently exit the housing 36 through the output optical fiber 50.
The attenuation level in this version of attenuator 34 adjusts as follows. The motor 38 activates to rotate the drive shaft 40 and subsequently the circular filter 42. As the circular filter 42 rotates, the various levels of attenuation pass in front of the light signal as it exits from input collimator 46 and enters the output collimator 48 and subsequently, the output optical fiber 50.
As illustrated in FIG. 1E, the circular filter 42 is a neutral density filter. The filter 42 has circularly varying attenuation levels along radians of the circle structure. The relationship of attenuation level to rotation of the circular filter 42 is illustrated in FIG. 1F. As can be seen, this relationship is also substantially linear. One concern in this type of optical attenuator 34 is that there exists a significant relative cost of forming the circularly varying attenuation levels of circular filter 42, in a predictable, monotonically increasing, fashion.
For the foregoing reasons, there exists in the art a need for a variable optical attenuator that is both cost efficient to manufacture and mechanically stable and reliable. The present invention is directed toward further solutions in this art.
In accordance with example embodiments of the present invention, a variable optical attenuator is provided having a housing. A motor mounts within the housing, and a drive shaft extends from the motor. A filter is mounted on the drive shaft of the motor such that the drive shaft passes through a substantially center point of the filter. The filter has a filter gradient that begins at a lower optical density (appears more clear) first edge of the filter and gradually increases in optical density (appears more opaque) toward a second edge of the filter, the second edge being diametrically opposed from the first edge. The filter gradient can be linear, substantially linear, monotonically increasing, and the like. An input optical fiber provides a light signal to be attenuated. The input optical fiber introduces the light signal, which passes through a first collimator, through the filter, through a second collimator, and to an output optical fiber which exits the housing. When activated, the motor rotates the drive shaft to position the filter to a desired attenuation position for attenuating the light signal.
In one aspect of the present invention, the filter element is a neutral density filter, and is substantially circular in shape. In still another aspect of the present invention, the filter element has a linear filter gradient, which gradually increases in optical density from a first edge of the filter to a second, diametrically opposed, edge of the filter.
In still another aspect of the present invention, the housing is sealed to prevent unwanted and undesired light from entering the housing.
In yet another aspect of the present invention, a potentiometer is provided within the housing. The potentiometer is in communication with the drive shaft to aid in determining the rotational position of the drive shaft.
In still another aspect of the present invention, a surface of the filter has placed thereupon, an entirely reflective coating to prevent stray light from interfering with the light signal. In yet another aspect of the present invention, the input optical fiber and collimator, and the output optical and collimator, are angled with respect to each other such that a reflection from the filter element of the light signal is not received in either of the input or output optical fibers or collimators.